1. Field of the Invention
The present invention relates to an image sensor which may be used in, for example, an image processing apparatus. More particularly, the present invention relates to an image sensor which is suitable for use in an apparatus that utilizes illuminating light of high coherence, e.g., laser light.
2. Description of the Related Art
FIG. 8 shows an external appearance of a CCD image sensor, which is a typical image sensor, and FIG. 9 shows a cross-sectional structure of the image sensor shown in FIG. 8. Referring to FIGS. 8 and 9, a CCD image sensing element (hereinafter referred to as "light-receiving element") 1 is formed with a light-receiving photodiode array or the like. The light-receiving element 1 is accommodated in a package 3 formed of a resin or ceramic material. The package 3 is substantially closed up with a cover glass (hereinafter referred to as "lid") 2 for protecting the light-receiving element 1. The lid 2 is a parallel-sided flat plate glass of about 1 mm in thickness, which is secured to the package 3 approximately parallel (horizontally) to the surface of the light-receiving element 1.
In general, both the obverse and reverse surfaces of the lid 2 are formed with anti-reflection coatings (thin film coatings) for preventing reflection of light in the wavelength region of illuminating light which is to be detected by the image sensor in order to reduce reflection loss at the glass surfaces (i.e., about 4% per surface). Thus, the surface reflectivity is held down to less than 1% per surface. On the other hand, the surface of the light-receiving element 1 is not particularly provided with anti-reflection coating aimed at a specific wavelength. Therefore, if the light-receiving element 1 is a device fabricated on a silicon wafer, for example, the surface of the element 1 has a surface reflectivity of about 50 to 60% for light in the wavelength range of from the visible region to the ultraviolet region.
However, in a case where an image sensor having the above-described arrangement is used in an apparatus designed to observe an object by utilizing substantially monochromatic illuminating light of high coherence, e.g., laser light, interference fringes are superimposed on an image which is to be observed, constituting an obstacle to observation or image processing and measurement. The way in which interference fringes are formed on the light-receiving element 1 will be explained below in detail with reference to FIG. 10. For the sake of simplicity, it is assumed in FIG. 10 that the lid 2 is a plane 2', and the light-receiving element 1 has a flat surface 1' without projections or the like, and that the two members are fixed at a very small angle of inclination .theta..degree. (unit: rad) with respect to each other.
The image sensor receives light passing through the lid 2 with the light-receiving element 1. Accordingly, as shown in FIG. 10, interference fringes are formed on the light-receiving surface 1' by interference between a light beam 4, which passes through the lid surface 2' and is reflected at the light-receiving element surface (light-receiving surface) 1' and further reflected at the lid surface 2' and then reaches the light-receiving surface 1', and a light beam 5 which passes through the lid surface 2' and directly reaches the light-receiving surface 1'. Although in FIG. 10 and the figures used in the following description, the light beams 4 and 5 are shown at a tilt to the light-receiving clement surface 1' for the sake of convenience, it should be noted that the light beams 4 and 5 are approximately perpendicularly incident on the light-receiving surface 1' in actual practice. FIG. 11 shows the contents of FIG. 10 in more detail. As shown in FIG. 11, the light beam 4 is incident on the lid 2' and perpendicularly reflected at the light-receiving element surface 1' and further reflected at the lid surface 2' at an angle of 2.theta..degree. and then incident on the light-receiving element surface 1' at the angle of 2.theta..degree., as has been described above. This is because the lid surface 2' is inclined at the angle of .theta..degree..
The interference fringes are known as Fizeau fringes, and the fringe contrast depends on 1 the surface reflectivity R.sub.L of the lid surface, 2 the surface reflectivity R.sub.I of the light-receiving element, and 3 the coherence of the laser light used. The pitch p of the interference fringes is determined by a relationship such as that shown in FIG. 12. Referring to FIG. 12, the light beam 4 (i.e., the wavefront thereof) passes through the lid 2 and is reflected at the light-receiving element surface 1' and also at the lid surface, inclined at the angle of .theta..degree., and then incident on the light-receiving element surface 1' at an angle of 2.theta..degree., as shown in FIG. 11. The light beam 5 (i.e., the wavefront thereof) is directly incident on the light-receiving element surface 1' approximately perpendicularly. The wavelength of the light beams 4 and 5 is represented by the interval .lambda. at either wavefront.
Assuming that a certain phase of interference fringes, for example, the center of a dark line, lies at a position 10 on the light-receiving element surface 1' and the center of a dark line which is adjacent to the first-mentioned dark line lies at a position 11. Therefore, as will be clear from FIG. 12, the distance between the two points, that is, the interference fringe pitch p, may be expressed by EQU p=.lambda./sin2.theta..degree..apprxeq..lambda./2.theta..degree.(1&gt;&gt;2.theta ..degree.).
Accordingly, the interference fringes are produced on the light-receiving element surface 1' in such a manner as to extend in a direction normal to the plane of FIG. 12 (the array direction is parallel to the plane of the figure). FIG. 13 is a perspective view of what is shown in FIG. 10, in which the interference fringes are represented by the lines 6.
Although in the foregoing description the lid surface 2' and the light-receiving clement surface 1' are assumed to be perfect planes, in actual practice these surfaces are not perfect planes but slightly undulated relative to each other due to the manufacturing accuracy of the lid 2, for example, or distortion caused by stress applied to the lid 2 when bonded to the package 3, or distortion occurring during the process of producing the light-receiving element 1 or when the light-receiving element 1 is attached to the package 3. Therefore, the interference fringes are correspondingly distorted. From a different point of view, it may be considered that the very small inclination angle .theta..degree. shown in FIG. 10 locally represents the relative undulation, that is, inclination, of the lid surface and the light-receiving element surface.